Schistosoma mansoni antigens play a crucial role in the induction of immunopathological processes and in modulating the host immune system. A polyclonal rabbit antiserum to an antigenic fraction of adult schistosoma worms was used to localize worm and egg antigens in tissue sections of infected mice. Most granuloma formations identified in paraffin sections of portal tracts and intestinal mucosa were vigorous with florid cellular composition, consisting of macrophages and epithelioid cells, surrounding a central nidus of S. mansoni egg. Schistosome pigments were demonstrated at the periphery of the granuloma, within sinusoidal Kupffer's cells and within macrophages of intestinal mucosa. Large amounts of schistosome antigen were found sequestered in the mesenteric lymph nodes and in the spleen. A strong reaction for locally synthesized IgG was found. These findings suggest that the schistosome antigen-antibody complex found in the host lymphoreticular system plays a major role in the immunopathological process.